nvme it's for the detection of SSD
i915 it's about the intel chipset
dm-crypt it's about the encrypted disk (If you have one if you don't it's not necessary to use this command line)

Finally we need to reconfigure the Kernel with this 3 lines or 2 depends of your configuration

Code:

sudo update-initramfs -k 4.4.0-040400-generic -u

It's very important to do -k instead -all I saw early in this post. -k will edit only the kernel 4.4 and don't touch to the kernel 4.2 that is working at the moment in your laptop. So if something wrong with 4.4 you will be happy to come back to 4.2.

That's all reboot and go to 4.4. Enjoy

More command to manage kernel is like me you try a lot :
"dpkg -l | grep linux-image" to see all kernel installed or removed in the laptop (rc=deleted, ii= installed you can delete it)
always keep the last kernel that working (4.2) and try to install 4.4.
"sudo dpkg --remove linux-imagexxxxxxx" pour supprimer un kernel installé (le passer de ii à rc)
"uname -r" to know the kernel that working now and don't delete this one.

Last thing I bought the Mouse Dell WM615 but I can't install it (it's a blueetooh mouse) if you have an idea.
And I can't keep my full resolution (UHD) on my laptop because Ubuntu can't manage two monitor with different resolution. It's working but it's so big for external monitor(1920*1080) and so small in the internal monitor. So I put both the same resolution (1920*1080). Is it possible to do something in Ubuntu for that ?

For information my firmware for video card is : xserver-xorg-video-nouveau

Finally Ubuntu 15.10 working fine with my new Dell XPS 9550 (wifi, hdmi with external monitor)
...Last thing I bought the Mouse Dell WM615 but I can't install it (it's a blueetooh mouse) if you have an idea.
...

I've struggled with a Logitech MX Anywhere 2 Bluetooth mouse before, and got it to work eventually. Maybe it will work for your mouse as well, see here.

Thanks for the help. I can finally use 4.4 kernel peacefully.
A small comment, you still did not fix the typos as mentioned by svea2. It should be 4.4.0-040400-generic and not 4.4.0.040400-generic in the update-initramfs command.

Thanks for the help. I can finally use 4.4 kernel peacefully.
A small comment, you still did not fix the typos as mentioned by svea2. It should be 4.4.0-040400-generic and not 4.4.0.040400-generic in the update-initramfs command.

And I can't keep my full resolution (UHD) on my laptop because Ubuntu can't manage two monitor with different resolution. It's working but it's so big for external monitor(1920*1080) and so small in the internal monitor. So I put both the same resolution (1920*1080). Is it possible to do something in Ubuntu for that ?

For information my firmware for video card is : xserver-xorg-video-nouveau

The issue is about the scale for menu and title bars.
If I put 1920x1080 on my external monitor and 3840x2160 on my laptop, it's good I can do it on Unity.
But If I open Firefox on the both, the font will be good for external and too small to my laptop.
For that when I have only one monitor I'm using the scale for menu and title bars to x2 and it's good.
But with two monitor if I do that the scale is shared between the two monitor so the laptop will be good in firefox and too big for the external monitor.
It will be nice to have different value for the scale, one for each monitor.